The present invention relates to a process for the treatment of silicon steel; in particular it relates to a process for transforming a sheet of grain oriented silicon steel, wherein an initial controlled amount of precipitates (sulfides and aluminum as nitride) is produced in the hot-rolled strip in a fine and uniformly distributed form, suitable for the control of the grain size during decarburization annealing; the control of the subsequent secondary recrystallisation is obtained by adding to the initial precipitates further aluminum as nitride, directly obtained in a continuous high-temperature treatment.
Grain oriented silicon steel for electrical applications is generally classified into two categories, basically differing in the level of induction, measured under the influence of a magnetic field of 800 As/m, this parameter being indicated as xe2x80x98B800xe2x80x99. Conventional grain oriented steels have B800 levels lower than 1890 mT; high-permeability grain oriented steels have B800 higher than 1900 mT. Further subdivisions have been made according to the so-called core losses, expressed in W/kg.
The conventional grain oriented steel, introduced in the thirties and the super-oriented grain steel, industrially introduced in the second half of the sixties, are essentially used for the production of cores of electric transformers, the advantages of the super-oriented grain product being its higher permeability, allowing cores of lower dimensions, and its lesser losses, allowing energy saving.
The permeability of electrical steel sheets is a function of the orientation of the cubic, body-centred iron crystals (grains); the best theoretical orientation is the one showing one corner of the cube parallel to the rolling direction.
Certain suitably precipitated products (inhibitors) called second phases, reduce the mobility of the grain boundary. Their use allows to obtain the selective growth of grains having the desired orientation; the higher the dissolution temperature in the steel of these precipitates, the higher the uniformity of orientation, the better the magnetic features of the end product. In the oriented grain, the inhibitor consists essentially of manganese sulfides and/or selenides, whereas in the super-oriented grain the inhibition is produced by a number of precipitates comprising said sulfides and aluminum as nitride, also in a mixture with other elements, from now on being referred to as aluminum nitride.
Nevertheless, in the production of the grain oriented and grain super-oriented steel, during solidification of the liquid steel and cooling of resulting solid, the inhibitors are precipitated in a coarse form, unsuitable for the desired purposes; therefore they must be dissolved and reprecipitated in the correct form, and so maintained until the grain having the desired dimensions and orientation is obtained at the stage of final annealing, after the cold rolling to the desired thickness and the decarburization annealing, i.e. at the end of a complex and costly transformation process.
Clearly the production problems, essentially due to the difficulty of obtaining good yields and constant quality, are mainly due to the measures to be taken for maintaining the inhibitors in the required form and distribution during the whole steel transformation process. In the case of the super-oriented product, a new technology has been developed in order to overcome these problems, as described e.g. in U.S. Pat. No. 4,225,366 and in EP 339474; these documents show the production of the aluminum nitride suitable for controlling the grain growth, by nitriding the strip preferably after the cold rolling step.
In the latter patent, aluminum nitride, precipitated in a coarse form during the slow solidification and the following cooling of the steel, is kept in this state by using low heating temperatures of the thick stab (lower than 1280xc2x0 C., preferably lower than 1250xc2x0 C.) before the hot rolling step; after the decarburization annealing, nitrogen is introduced in the sheet (essentially in proximity of its faces); it then reacts by producing silicon- and manganese-silicon nitrides having a relatively low solubilization temperature, which are dissolved during the heating phase in the final box-annealing. Nitrogen released in this manner can now deeply penetrate the sheet and react with aluminum, reprecipitating in a fine and homogeneous form along the whole thickness of the strip in the form of mixed alluminum and silicon nitride; this process requires the permanence of the material at 700-800xc2x0 C. for at least four hours. In cited EP patent it is stated that the temperature of nitrogen introduction must be close to the decarburization temperature (about 850xc2x0 C.), and in any case not higher than 900xc2x0 C., in order to avoid an uncontrolled grain growth, given the absence of suitable inhibitors. In fact, the optimal nitriding temperature appears to be 750xc2x0 C., whereas 850xc2x0 C. represents the upper limit to avoid such uncontrolled growth.
This process seems to comprise certain advantages, such as the relatively low heating temperature of the slab before the hot rolling step, or the relatively low decarburization and nitriding temperatures; another advantage lies in the fact that there is no increase in production costs in maintaining the strip in the box-annealing furnace at 700-800xc2x0 C. for at least four hours (with the purpose of obtaining the mixed aluminum and silicon nitrides necessary for a controlled grain growth), because the time required for heating the box annealing furnaces is approximately the same.
However the above cited advantages are associated to some disadvantages, among which: (i) the almost total lack of precipitates inhibiting the grain growth, due to the low heating temperature of the slab; as a consequence, any heating of the strip, i.e. during the decarburization and nitriding processes, has to be performed at relatively low and critically controlled temperatures, in order to prevent an uncontrolled grain growth under the above referred conditions; (ii) the impossibility of taking any measures during the final annealing step, in order to accelerate the heating time, e.g. by replacing the box-annealing furnaces with other furnaces operating in continuous.
The present invention aims at overcoming the disadvantages of the known production systems, by proposing a new process allowing the control within optimal limits of the size of the grain of primary crystallisation and, at the same time, allowing to perform a high-temperature nitriding reaction enabling the correction of the total useful inhibition content, up to the necessary values, directly during continuous annealing.
According to the invention, the continuously cast slab is heated at a temperature sufficient to dissolve a limited but significant amount of second phases like sulfides and nitrides, which are thereafter reprecipitated in a way suitable to control the grain growth up to the decarburization annealing, included. In the course of a further high-temperature treatment during the same continuous annealing, further aluminum-bonded nitrogen is precipitated, in order to adapt the total amount of second phases to the desired grain orientation during the secondary recrystallisation.
The present invention relates to a process for the production of an electrical steel sheet, wherein a silicon steel is continuously cast, hot-rolled and cold-rolled, and wherein the obtained cold strip is annealed in continuous in order to perform primary recrystallisation, decarburization, and thereafter (still under continuous conditions) nitriding, coated with an annealing separator, and box-annealed in order to perform a final secondary crystallisation treatment, said process being characterised by the combination in cooperation relationship of the following steps:
(i) producing a hot-rolled sheet in which the inhibition level (Iz) necessary to control the grain growth, calculated according to the empiric formula:
Iz=1.91 Fv/r
(where Fv is the volumetric fraction of the useful precipitates and r is their mean radius) is comprised between 400 and 1300 cmxe2x88x921; this can be done for instance by performing an equalising thermic treatment onto the continuously cast steel at a temperature comprised between 1100 and 1320xc2x0 C., preferably between 1270 and 1310xc2x0 C., followed by hot-rolling under controlled conditions;
(ii) performing a continuous primary recrystallisation annealing of the cold-rolled strip at a temperature comprised between 800 and 950xc2x0 C., in a wet nitrogen-hydrogen atmosphere, said annealing optionally comprising a decarburization step;
(iii) performing under continuous conditions a nitriding annealing step at a temperature comprised between 850 and 1050xc2x0 C., for a time comprised between 5 and 120 s, by introducing in a nitriding area of the furnace some nitriding, preferably NH3containing gas in a quantity of between 1 and 35 normal liters per kg of treated strip, together with steam in a quantity between 0.5 and 100 g/m3, the NH3 content of said gas preferably being comprised between 1 and 9 normal liters per Kg of treated steel.
According to the present invention, it is also possible to remarkably increase, during the next secondary recrystallisation treatment, the heating rate within the temperature range of 700 to 1200xc2x0 C., thereby reducing the heating time from the conventional 25 hours or more, necessary according to the known processes, to less than four hours; interestingly, this is the same temperature range as critically required by the known processes in order to dissolve the silicon nitride formed on the surface, to diffuse the released nitrogen into the sheet, and to form a precipitate consisting of mixed alluminum nitrides, such process requiring, according to the known teachings, at least four hours at a temperature comprised between 700 and 800xc2x0 C.
As far as the steel composition is concerned, alluminum should suitably be present in the range of 150 to 450 ppm.
Besides, it should be noted that it is not necessary to perform the nitriding treatment after the primary recrystallisation: it may also be performed during other steps of the transformation process of the laminate after the cold-rolling step.
Of course, the remaining part of the transformation cycle is performed according to specific modalities depending on the desired final product; these modalities will not be referred to in the description, unless when necessary for exemplification purpose.
The present invention allows, independently from the desired end product, to operate under no tight temperature control, and yet to obtain, in primary recrystallisation, a grain with optimal dimensions for the final quality; it also allows to obtain the direct high-temperature precipitation of aluminum as nitride during the nitriding annealing step.
The basis of the present invention can explained as follows. It is deemed necessary to maintain a certain amount of inhibitor in the steel up to the continuous nitriding annealing step; this amount should not be negligeable, and should be suitable to control the grain growth, thereby allowing to work at relatively high temperatures, avoiding at the same time the risk of an uncontrolled grain growth, with severe shortfalls in yields and magnetic qualities.
This can be obtained in several ways along the production cycle preceding the cold rolling step, for example by combining (a) a precise choice of composition elements necessary for the precipitation of sulfides, selenides, and nitrides, such as S, Se, N, Mn, Cu, Cr, Ti, V, Nb, B, etc., and/or elements which, when present in solid solution, may affect the movement of the grain boundary during the thermic treatments, such as Sn, Sb, Bi, etc. together with (b) the employed type and modality of casting, the temperature of the cast bodies before the hot rolling step, the temperature of the hot rolling step itself, the thermic cycle of the hot-rolled strips possible hot annealing.
Independently from their method of production, the final strips must show a useful inhibition content within a well defined range: on the basis of extensive experimentation performed in laboratory as well as on industrial plants, the present inventors have defined this range as being comprised between 400 and 1300 cmxe2x88x921 (as shown in Example 1 below).
During said experiments it was also found that the total inhibition value allowing to obtain the best magnetic features depends, case by case, on the grain size distribution developed during primary recrystallisation: the higher the grain mean size and the lower the standard deviation of the size distribution, the lower the inhibition level necessary for the grain control.
In the specific case of the present invention, the control of precipitates is obtained by maintaining the slab temperature high enough to solubilize a significant amount of inhibitors, but at the same time low enough to prevent the formation of liquid slag, thereby avoiding the need for expensive special furnaces.
The inhibitors, once finely reprecipitated after the hot-rolling process, allow to avoid an extended control of the treatment temperatures; they also allow to increase the nitriding temperature up to the level necessary for the direct precipitation of aluminum as nitride, and to increase the rate of nitrogen penetration and diffusion into the sheet.
The second phases present in the matrix work as nuclei for said precipitation induced by the nitrogen diffusion, also allowing to obtain a more uniform distribution of the absorbed nitrogen along the sheet thickness.